What are the key concepts and main troubleshooting tips and best practices when using Aspen HYSYS Dynamics?
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Aspen HYSYS Dynamics
Last Updated:
31-Jan-2018
Last Updated:
31-Jan-2018
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Article ID:
000094883
Article ID:
000094883
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Problem Statement
What are the key concepts and main troubleshooting tips and best practices when using Aspen HYSYS Dynamics?
Solution
A. Steady State Backup File.
Always save a copy of the steady state simulation file you will use to depart from steady state to dynamic mode, so whether something goes wrong or if any change(s) need(s) to be made in steady state, you can start off again from the same steady state file. Going back from dynamic mode to steady state most of the times will not lead to a solution, but to convergence issues..
B. Dynamic Solver.
1) The dynamic solver in Aspen HYSYS is not the same as the steady state solver.
2) The dynamic solver considers the flowsheet as a boundary value problem.
3) If either pressure or flow specs are made on all the boundary streams (i.e. feed(s) and product(s)), then the Pressure/Flow (P/F) solver will be able to solve all internal pressures and flows simultaneously. Dynamic calculations are not sequential as in steady state.
4) Three key concepts:
4.1) Pressure/Flow specs (only one, either pressure or flow) for boundary streams.
4.2) Resistance / Conductance operations (equations that calculate pressure drop, such as valves).
4.3) Pressure node operations (calculate a pressure, such as vessels).
5) All unit operations must be properly sized prior to moving from steady state to dynamic mode on either the 'Rating' or 'Dynamics' tab of each unit.
C. Fluid Package.
1) Aspen HYSYS Dynamics uses the same physical property calculation routines as Aspen HYSYS. Make sure that the selected Fluid Package is the best option.
2) In case of convergence difficulties or noise, try other combinations:
2.1) Activate the ‘Try IOFlash’ option.
2.2) Change to ‘Multi Phase’ method if there are 2 liquid phases.
3) Remove those components that are not required for the dynamic simulation, it is, those components for which composition is negligible or zero. The calculation of the properties of these components (with compositions equal to zero) can lead to convergence problems. An additional benefit is that the simulation size will decrease, hence it will run faster.
D. Troubleshooting Tips.
1) Always analyze the problem with Process Engineering judgement. It may be useful to study a large flowsheet in smaller sections.
2) Use spreadsheets and strip charts to track the behavior of process variables with respect to time.
3) Check the control system and its tuning parameters.
4) Use the ‘Event Scheduler’ to repeat actions in a consistent way, rather than manually changing the control system performance.
5) Use ‘snapshots’ to store your starting points and/or save a moment during the dynamic run if you need to go back to such moment again.
6) Pay attention to the control system if it is not operating properly.
6.1) Add controllers where required.
6.2) Double check controller settings, especially the controller action mode.
6.3) Suspect of saturated controllers.
6.4) Suspect of controllers with high gain, but short integral time.
7) The following are values that have been proved to work fine as tuning parameters for controllers. However, users are encouraged to make use of accurate information (provided by vendors) to rigorously model the performance of the control system.
8) You can only change the value of fixed variables (pressure/flow specs).
E. Best Practices.
1) Do not run your model in dynamic mode without a control system and proper tuning parameters.
2) A vapor phase will be needed in separators. Consider if you will need to add inert components to pressurize the vessel.
3) Do not connect two pressure nodes directly (e.g. separators). Use a resistance/conductance operation in between (e.g. valves).
4) Think about the suggestions made by the ‘Dynamics Assistant’. You do not have to necessarily make all the listed changes.
5) If static head contributions are considered, you will need to check the nozzles locations in all blocks.
Keywords
Dynamics, Best Practices, Troubleshooting, Tips.